The candidate is a productive young investigator firmly committed to an academic research career. Based on her findings while a research trainee, she has developed two new hypotheses regarding the mechanisms of canalicular bile formation and the pathogenesis of cholestasis. These hypotheses form the basis for this research proposal. First, while there is now compelling evidence that hepatic bile acid uptake is dependent upon a sodium-coupled uptake step driven by the electrochemical sodium gradient maintained by Na, K-ATPase, the relationship between Na,K-ATPase cation pumping and bile acid-dependent bile flow (BADBF) is poorly understood. The hypothesis that primary changes in Na,K-ATPase cation pumping play a role in cholestasis will be tested using, as probes, chlorpromazine and estrogen, compounds known to inhibit Na,K-ATPase in isolated membranes. The following questions will be addressed: 1) Do chlorpromazine and estrogen also inhibit cation pumping in the intact cell? 2) Do these agents diminish the electrochemical sodium gradient, thereby inhibiting sodium-coupled uptake of bile acids and BADBF sufficient to cause cholestasis? Second, previous findings suggest that HCO3- secretion accounts for much of bile acid-independent bile flow (BAIBF). The candidate postulates that hepatic HCO3- secretion is mediated, at least in part, by a plasma membrane proton translocating ATPase (H+ ATPase). This proposal will look for such a H+ ATPase, characterize its function and examine its role in BAIBF and certain forms of cholestasis. These hypotheses will be tested using cultured rat hepatocytes, liver plasma membrane vesicles and the perfused rat liver, techniques already established in this laboratory. The findings will provide important new insight into the mechanisms of canalicular bile formation and the pathophysiology of cholestasis, as well as information regarding poorly understood aspects of epithelial function in general, including: 1) the functional relationships between Na,K-ATPase cation pumping, sodium-coupled solute uptake and modulation of epithelial transport, and 2) the role of active H+ transport in the sponsorship of Dr. Bruce Scharschmidt within the stimulating and productive research environment of UCSF and the Liver Center where the candidate will interact with a community of active investigators engaged in the study of epithelial transport and hepatic physiology.